Conversion of the substance to be detected takes place in electrochemical sensors during the measurement of gases, and the conversion can be evaluated in a qualified manner on the basis of changes in potential at the measuring electrode (frequently called working electrode) and/or due to currents flowing through the electrochemical sensor. The information content of the measurement results obtained can be considerably increased if a third electrode is used as a reference electrode, which is the case in common three-electrode sensors. This third electrode has no current load, as a result of which a so-called “open circuit potential” (OCP) is formed at this electrode. This OCP is used as a reference potential for one or more measuring electrodes. The sensors are actuated mostly by so-called potentiostats, which maintain the potential difference between the measuring electrode and the reference electrode at a constant value, while the current flow is measured between the measuring electrode and the auxiliary electrode.
To obtain the “open circuit potential” as a reliable reference potential, it is to be ensured that processes taking place in the sensor will affect this reference potential as little as possible. Such an effect can occur due to various processes. For example, reaction products formed at the measuring electrode or auxiliary electrode may diffuse to the reference electrode and cause changes in the reference potential. H+ and OH− can be primarily considered to be such reaction products. So-called mediators are frequently also used in electrochemical sensors. Mediators are ions or molecules that transfer electrons between the analyte and electrodes and are usually selected to be such that they react with the analyte as selectively as possible. Reduced or oxidized mediator molecules or ions, which diffuse into the vicinity of the reference electrode and may lead to potential change there, also occur in this case. Analyte and/or interfering accompanying gases may reach the reference electrode in case of high gas concentrations and bring about a corresponding change in potential. This risk occurs, for example, in case of high CO concentrations as a consequence of fires. The decomposition of organic electrolyte solution may likewise lead to disturbances and long-lasting poisoning of the reference electrode.
Besides general requirements on a reference electrode, such as the simplest manufacture possible, or environmental friendliness, the lowest possible toxicity and good ability to be disposed of, a number of electrochemical requirements are to be imposed on reference electrodes. These include the stable design of the electrochemical reference potential and extensive independence of this reference potential from changes in the pH and pO2 values. Furthermore, the susceptibility to interferences and poisoning phenomena on the electrode surface, which can be caused by electrochemical reaction products or interfering gases from the electrolyte, must be as low as possible.
Various approaches to designing reference electrodes in three-electrode electrochemical sensors have been known, but they all meet the requirements described only partially.
A reference electrode of this class is known from DE 42 31 256 A1. The type of reference electrode disclosed there comprises a reference electrode made of a catalytically active precious metal, on the surface of which oxide is formed. Platinum/platinum oxide electrodes and iridium/iridium oxide electrodes are frequently used. These electrodes are characterized by simple manufacture and the stable design of a reference potential and are nontoxic or only slightly toxic. Their drawback is the high susceptibility to poisoning phenomena as well as the possibility that the reference potential can be affected by changing pO2 and pH values in the electrolyte environment.
Other known reference electrodes according to the state of the art are so-called reference electrodes of the first kind, in which a metal or a soluble phase is at equilibrium with a corresponding ion. The best-known example of this is the so-called hydrogen electrode.
Another approach from the state of the art is the use of a reference electrode of the second kind, which is characterized by a metal, which is at equilibrium with a poorly soluble metal salt. Mercury/mercury sulfate electrodes (GB 2 066 965 A) and silver/silver chloride electrodes (WO 99 01 757 A1) shall be mentioned as examples here. All the above-mentioned reference electrodes according to the state of the art have, on the whole, an excessively strong tendency to show interference phenomena with toxic gases.